1. Field of the Invention
This invention generally relates to electric lamps and has particular reference to an improved high-pressure mercury-vapor dischare lamp having integral means for modifying the color-rendering characteristics and chromaticity of its light output.
2. Description of the Prior Art
High-pressure mercury-vapor discharge (HPMV) lamps are well known in the art and employ arc tubes that generate radiations of various intensities in the ultraviolet and visible portions of the spectrum. In the visible region, the major line emissions occur at 405, 436, 546 and 578 nanometers (nm.). The most intense radiations in the ultraviolet region occur at 253, 296 and 365 nm. The light output of such lamps is thus deficient in red radiations (beyond 600 nm). As a result, their illumination characteristically has a bluish tint and distorts the colors of most objects.
In order to improve the color-rendering properties and source color of such lamps, it became the standard practice to coat the outer protective envelope with a phosphor that absorbed some of the ultraviolet radiation and converted it into red radiation. A HPMV lamp using an interior phosphor coating of manganese-activated magnesium fluorogermanate is disclosed in U.S. Pat. No. 2,748,303, issued May 1956 to Thorington. Another red-emitting phoshor used for this purpose is tin-activated strontium orthophosphate phosphor described in U.S. Pat. No. 3,110,680, issued November 1963 to Koelmans et al.
Further improvement is the ability of high-pressure mercury-vapor lamps to portray lighted objects in their true colors was achieved by coating the outer envelope with a copper-activated strontium phosphate type phosphor that emitted blue or green-blue radiations, and by mixing such a phosphor with red-emitting manganese fluorogermanate phosphor to provide a single layer of blended phosphors. A HPMV discharge lamp utilizing a luminescent coating of such phosphors is described in U.S. Pat. No. 3,328,620 issued June 1967 to Rimbach.
A high pressure mercury lamp utilizing a color-modifying coating of terbium-doped europium-activated yttrium vanadate phosphor which emits red radiation is disclosed in U.S. Pat. No. 3,480,819 issued November 1969 to Faria et al. Dysprosium-activated ytrrium vanadate type phosphors which employ various coactivators and emit in the red, yellow and blue-green regions of the spectrum have been proposed for use in HPMV lamps to improve their light output (U.S. Pat. No. 3,555,337 issued January 1971 to Faria et al).
The use of europium-activated yttrium vanadate phosphor as a color-modifying coating for HPMV lamps which adds orange-red radiations is disclosed in U.S. Pat. No. 3,569,762, issued Mar. 1971 to Levine et al. An article by Luscher and Datta in the January 1970 issue of "Illuminating Engineering" (Vol. 65, No. 1, pp. 49-53) also discloses the use of this phosphor, as well as red-emitting yttrium vanadate phosphate type phosphors, in HPMV lamps to improve their light output.
According to more recent developments in this field, the color rendition of HPMV type discharge lamps is enhanced by combining a blue-violet emitting phosphor (such as europium-activated strontium chlorophosphate) and an orange-red emitting phosphor (such as europium-activated ytrrium phosphate vanadate) and coating them onto the outer envelope in either two separate layers or as a single layer in the form of a two-component blend. A lamp of this type is disclosed in U.S. Pat. No. 3,670,194 issued June 1972 to Thornton et al.
Another modified HPMV lamp employs a coating on the outer envelope which consists of blended europium-activated strontium chlorosilicate and europium-activated yttrium vanadate phosphate phosphor which absorb ultraviolet radiation and emit in the blue-green and red portions of the spectrum, respectively. (U.S. Pat. No. 3,790,490 issued February 1974 to Datta et al).
A color-corrected HPMV lamp utilizing an outer envelope that is first coated with a non-luminescent coating of silica, titania, magnesia or alumina and then with a phosphor coating consisting of a blend of yttrium vanadate (or yttrium phosphate-vanadate) and magnesium fluorogermanate (or magnesium arsenate) which converts some of the ultraviolet radiation into red or orange-red emission and provides a light source having incandescent characteristics is described in U.S. Pat. No. 3,825,792 issued July 1974 to Rokosz et al.
Modification of a HPMV lamp to improve its color rendition and provide a color temperature in the range of 3200K to 4500K by coating the outer envelopes with a blend a blue-green emitting phosphor (europium-activated strontium chlorosilicate) and a red-emitting phosphor (europium-activated yttrium vanadate phosphate) which converts ultraviolet radiations into visible radiations of the respective colors is disclosed in U.S. Pat. No. 3,866,083 issued February 1975 to Datta et al.
The use of a sensitized cerium-activated calcium sulfide phosphor in a HPMV lamp to convert ultraviolet radiation into color-correcting visible radiation is suggested in U.S. Pat. No. 3,898,174 issued August 1975 to W. Lehmann.
While the prior art lamps greatly improved the color-rendering characteristics of HPMV lamps, the spectral distribution of their light outputs is such that color distortion of various articles illuminated by such lamps is still present to such a degree that the use of such lamps for interior lighting in department stores and similar applications is not commercially attractive. The color distortion is quite pronounced in the case of gold, blue and red articles. The golds are rendered more yellow-green, the blues more violet and the reds more purple -- none of which is acceptable to a disconcerning department store shopper. It would accordingly be very desirable to provide a light source that has the high efficacy and long life of a HPMV discharge lamp and which also has a source-color and color-rendering characteristics which are similar to those of an incandescent lamp.